Move sysinst sources to usr.sbin.
This commit only physically moves the sources - there are no other
changes, to maximize the probability that this will be treated as a
rename if we ever do manage to migrate away from CVS.
Moving sysinst has been discussed on and off for years and has two
goals: making it easier to work on sysinst, and also making sysinst
available on running systems for use installing chroots and VM images
and other such things. None of the latter is possible yet, but as they
say, one thing at a time.
Doing this now was approved in an impromptu fashion by mrg, riz,
riastradh, me, and groo.